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A Case of Olkaria Geothermal System, Kenya

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A Case of Olkaria Geothermal System, Kenya International Journal of Scientific Engineering and Applied Science (IJSEAS) – Volume-7, Issue-3, March2021 ISSN: 2395-3470 www.ijseas.com Application of MT and TM Resistivity Techniques in a High Temperature Geothermal System: A case of Olkaria Geothermal System, Kenya Solomon Namaswa1, 3, John Githiri1, Nicholas Mariita2, Maurice O. k’Orowe1 and Emmanuel Rionomakal4 1Physics Department, Jomo Kenyatta University of Agriculture and Technology, Kenya 2Geothermal Training and Research Institute, Dedan Kimathi University of Technology, Kenya 3Physics Department, Multimedia University of Kenya 4 Electrical and Electronic Engineering Department, Nairobi Technical Training Institute, Kenya Abstract The most important aspect in geothermal energy exploration is the ability to predict the location and extent of the subsurface resource. This has been achieved by application of geophysical methods due to its ability to image subsurface structures. The electrical resistivity of the earth’s subsurface is affected by geothermal processes as a result of fracturing that increases fluids and rock interaction leading to the formation of alteration minerals that create subsurface resistivity contrast as a function of depth and temperature. A combined TEM and MT soundings was carried out in Olkaria domes geothermal field to map subsurface resistivity distribution. The MT static shift was corrected by TEM data through a joint inversion of 120 MT and 79 TEM soundings. From the resistivity contours at selected depths, three main resistivity regions were inferred; one low resistivity to the Northwest, the second low resistivity was observed to the Eastern side of the field. The two low resistive regions are separated by a NE-SW trending high resistive region. Resistivity decreases with depth up to a depth of 500masl then it increases with depth. From the cross sections, the results reveal three main resistivity zones. The first zone was characterized by a narrow layer of higher resistivity near the surface and was depicted to represent unaltered region. This was followed by another broader layer of high conductivity that was interpreted to be due to high conductive hydrothermally altered mineralogy such as zeolites. A relatively higher-resistivity zone follows at which resistivity is controlled by the formation of high temperature mineralogy at depth such as epidote. It is recommended that directional production wells can be drilled in the western and eastern regions where resistivity is low, and believed to represent permeable zones. The wells should bend towards the centre at depth where resistivity is high representing high temperature core. Key words: MT, TEM, Resistivity, Geothermal Field, Olkaria Domes 1.0 Introduction The greater Olkaria geothermal area is situated in the southern part of the Kenyan rift, nearly 127 km from Nairobi (0° 53’S; 36° 18’E) as shown in Figure 1. The Kenyan rift forms part of the structure of the East African rift that extends to Mozambique region in the south from Ethiopia (Kandie et al., 2016). It forms a section of the eastern arm that extends to Lake Natron from Lake Turkana, (Omenda et al., 2016). 100 International Journal of Scientific Engineering and Applied Science (IJSEAS) – Volume-7, Issue-3, March2021 ISSN: 2395-3470 www.ijseas.com Figure 1: Location of Olkaria Geothermal Field in the Kenyan Rift Valley (Wanjohi, 2014) Geophysical method of resistivity is used to show the resistivity changes beneath the earth’s surface. This method has found its application in the expedition of geothermal energy since good geothermal reservoir has been associated with low resistivity (Abdou, 2015). Electrical conductivity of rocks rely on rock permeability, porosity, fluid salinity, temperature and pressure (Harald, et al., 2020). The MT and TEM geophysical methods have played a major role in the X-Raying and detailing the buried earth’s interior by means of establishing structures that are deeply seated and are in charge of geothermal system control. These structures include heat sources and geothermal fluid flow conduits (Arthur, 2017). Geophysical methods of study have played an important aspect in understanding earth’s interior as they are the only mechanisms of establishing subsurface structures at depth at a reduced cost as compared to direct drilling method (Wamriew, 2019). Determination of subsurface signatures from resistivity method is achieved by the ground being actuated by currents and the signal produced observed at the surface. This process measures apparent resistivity which needs to be transformed into specific resistivity that is material dependent. Rocks are regarded as poor conductors and their conductivity can only be achieved if their properties are transformed through alteration by earth processes. Therefore, porosity and 101 International Journal of Scientific Engineering and Applied Science (IJSEAS) – Volume-7, Issue-3, March2021 ISSN: 2395-3470 www.ijseas.com fluid conductivity are the major reasons for rock conductivity near earth surfaces (Thanh, et al., 2019). 2.0 Field Setup and Data Acquisition 2.1 MT Data Acquisition The MT data were collected by Kenya electricity generating company (KENGEN) between 2005 and 2017 by use of a 5-channel (MTU-5A) MT equipment. The equipment layout was as shown in figure 2. MT data from each site was acquired for about 20 hrs. Figure 2: MT Equipment Setup (Flóvenz Et Al., 2012) 120 MT soundings were used for interpretation in the Domes prospect covering an area of about 45 km2 as shown in figure 3. Figure 3: Magnetotelluric Soundings 102 International Journal of Scientific Engineering and Applied Science (IJSEAS) – Volume-7, Issue-3, March2021 ISSN: 2395-3470 www.ijseas.com 2.2 Transient Electromagnetic (TEM) Method The TEM equipment used in this survey is from Zonge. A 200 m × 200 m transmitter wire loop was used (Figure 4) Figure 4: Transient Electromagnetic Equipment Setup [7] A total of 79 Central Loop TEM soundings were carried out and were distributed as shown in figure 5. Figure 5: Transient Electromagnetic soundings 2.3 MT Data Processing The SSMT2000 software was used to process the data in time-series form from the MT. through Fourier transformation, the data was changed to frequency spectrum. Noisy data points were eliminated by the MT editor. The resulting time-series data were Fourier transformed to the 103 International Journal of Scientific Engineering and Applied Science (IJSEAS) – Volume-7, Issue-3, March2021 ISSN: 2395-3470 www.ijseas.com frequency domain. From the Fourier transform band, the robust processing method was used to compute average cross-powers and auto powers, which were then edited to get rid of noisy data by the MT editor program graphically. The data was then saved in Electronic Data Interchange (EDI) files and later used as TEMTD input for joint inversion with the TEM. 2.4 TEM Data Processing The raw TEM data processed using the program TemxZ where the data collected at same were averaged and late time apparent resistivity computed as a function of time after current turn-off. 2.5 The MT Static Shift Analysis The MT static shift corrected by TEM data through joint inversion as shown in figure 6 Figure 6: joint 1-D inversion of TEM and MT soundings for station DMT08A 3.0 Results 3.1 Resistivity Iso Maps Contours maps at different elevations from 1900 m.a.s.l down to 2000 m.b.s.l were constructed by TEMRESD program. 3.1.1 Resistivity map at 1900 m.a.s.l contours This is shown in figures 7 and covers a depth of 100 metres below the surface. The figures show a dominant high resistivity region with resistivity ranging from 16Ωm-1000Ωm. 104 International Journal of Scientific Engineering and Applied Science (IJSEAS) – Volume-7, Issue-3, March2021 ISSN: 2395-3470 www.ijseas.com Figure 7: Resistivity Iso Map at 1900 masl Figure 8: Geological Map (Mariita, 2009) High resistivity in this region is an indication of un-altered subsurface formations near the earth’s surface. The eastern region of the field exhibits the highest resistivity trending northwards and in the SW direction. This outlines the ring structure as indicated in the geological map (figure 8). This could be as a result of resistive rhyolite rocks that cover the top parts of the structure. The central part shows lower resistivity that shifts towards the west and northwest parts as we move down the surface. The general resistivity within the field gradually reduces with depth as shown in figure 9. 3.1.2 Resistivity maps at 1500- 1000 m.a.s.l contours This is shown in figures 9 and 10 and covers a depth of 500 to 1700 metres below the surface. The figures show a region of enhanced conductivity ranging from 1Ω-80Ω across the area especially in the western and eastern regions. 105 International Journal of Scientific Engineering and Applied Science (IJSEAS) – Volume-7, Issue-3, March2021 ISSN: 2395-3470 www.ijseas.com Figure 10: Resistivity Iso Map at 1000 masl Figure 9: Resistivity Iso Map at 1500 masl Relatively high resistive anomaly patches appears to emerge at the centre of the study area aligning in the NE - SW direction. This indicates a structural control that coincides with the geological fault structure in figure 8. This high resistivity anomaly could be as a result of un- altered or presence of clay cap fills in the fault line. High resistivity is observed in the SW region. This indicates patches of un-altered grounds at this depth. The western and eastern flanks have low resistivity also aligning in the NE - SW direction. This is as a result of high permeability as shown in temperature recovery profiles (Figure 11) and low resistivity hydrothermal alteration minerals such as zeolites and smectites with their cations loosely bound are formed making the rocks conductive hence an indication of high hydrothermal fluid circulation exists at depth. The trend of resistivity decreasing with depth continues.
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